1. Field of the Invention
The present invention relates to a method for producing optical fiber coated with a carbon layer.
2. Prior Art
Quartz-based optical fibers have been widely used for communications cables. Hydrogen coming into contact with these fibers disperses therein, and the molecular vibrations of the hydrogen lead to greater absorption losses. In addition, the hydrogen diffused therein may react with P.sub.2 O.sub.3, GeO.sub.2, or B.sub.2 O.sub.3, which are contained in the fiber as dopants, and forming compounds with one or more OH groups. Absorption by the OH group also increases absorption losses. One way to solve these problems is to add a liquid-phase composition which can absorb hydrogen in the fiber (Japanese Patent Application kokai No. 61-251808). However, this method is impractical: the produced fiber has a limited capacity for hydrogen absorption and is structurally complex. Corning Glass (International Wire & Cable Symposium Proceedings 1987, pages 241-244, and Journal of Lightwave Technology, Vol. 6, No. 2, February 1988, pages 240-244) and AT&T (Electronic Letters, 13th Oct. 1988 Vol. 24, No. 21, pages 1323-1324, and OFC '88/Tuesday Afternoon/23) have recently disclosed that coating the fiber with carbon by chemical vapor deposition (CVD) can enhance its resistance to hydrogen. In the methods, an uncoated optical fiber prepared in a spinning furnace is led to a hot CVD furnace, and the hydrocarbon compounds are thermally decomposed to form a carbon layer on the surface of the uncoated optical fiber in the CVD furnace. In the hot CVD process, aqueous molecules absorbed on the surface of the uncoated fiber are however dispersed in the core of the uncoated optical fiber, and reacted with dopants which have been previously dispersed therein. Accordingly, transmission loss is greatly increased in the wavelength of 1.39 .mu.m depending on the absorption of OH groups.
Also, aqueous molecules absorbed on the surface of the optical fiber is reacted with the optical fiber at high temperature in steps of forming carbon-coated layer on the surface of the optical fiber to form silanol groups. Also, hydrogen radicals, which are produced by thermally decomposing the original hydrocarbon compound, cut the siloxane bonds of composition of the optical fiber to form silanol groups. The silanol groups erode the surface of the optical fiber to lead to degrade mechanical properties thereof.
Furthermore, aqueous molecules absorbed on the surface of the optical fiber lead to degrade deposition rate for carbon coating.